In the liquid cooling of heated surfaces operating at high heat fluxes, the required liquid velocities to avoid film boiling can be quite high. The result is that large pumps and high pressures are required. By employing concavely curved heat exchange surfaces two phase, i.e. boiling, heat transfer can be increased as compared to linear flow. It is desirable to further enhance heat transfer from concave curved heat exchange surfaces whereby even higher critical heat fluxes may be obtained.